The present invention relates to a positional sensing transducer which utilizes an elongated tube constructed of a magnetically saturable material. A sense wire runs through the tube parallel to the elongated axis of the tube and a pair of oppositely polled magnets are positioned on diametrically opposite exterior portions of the tube so that the magnetic flux from the magnets will completely saturate the portion of the tube that lies between the magnets, while the remaining portion of the tube remains unsaturated. The use of a ferrite element that is of a toroidal shape and is positioned between two saturating magnets to produce an output signal on a sense line that runs through the core is disclosed in U.S. Pat. No. 3,638,221, issued in the name of Victor M. Bernin on Jan. 25, 1972, and assigned to the assignee of the present invention. The apparatus of the Bernin patent is a keyboard switch in which the entire toroidal core is completely saturated when the magnets are lowered on a keystem into the vicinity of the cores. In order to accomplish the purposes of the Bernin patent, the core of the switch of that patent is positioned so that its axis is parallel to the wide part of the magnets which are of a substantially greater dimension than is the thickness of the core.
The present invention, by contrast, uses a hollow, elongated, cylindrical tube constructed of a material which is magnetically saturable, a sense wire that runs through the tube, and two oppositely polled magnets that move along the outside of the tube in order to provide an accurate linear indication of the position of the magnets with respect to the tube on the sense line. In other words, the present invention does not provide a "1" or a "0" output signal, but instead it may be used to accurately determine the position of the saturating magnets with respect to the tube. The portion of the elongated tube that lies between the magnets is saturated while the remaining portion is not. Because the tube provides a closed flux path, there is no substantial fringing affect at the ends of the saturating magnets; and, therefore, the portion of the tube that is not between the magnets remains substantially unsaturated. Since the output signal that is provided on the sense wire through the tube is not dependent upon the magnetic characteristics of the tube, but merely upon the position of the magnets with respect to the tube, a very linear output signal is achieved. In addition, problems that affect magnetic sensors that depend on partial saturation of the sensing element, such as temperature variation and aging variation, are also eliminated. Moreover, magnetic force that is required to operate the device of the present invention is not critical because of reliance on saturation of the tube between the magnets to produce the output signal. This is in direct contrast to sensing devices such as those shown in the McAdam patent No. 2,915,637 where the magnetic saturation of the entire toroidal core is affected by the position of the adjacent magnet; and, thus, the core is subject to the previously mentioned temperature and aging defects; and the device has a critical magnetic force requirement for the control magnet.